2,4-Dichlorophenoxyacetic acid (2,4-D) is a herbicide used to control broadleaf weeds. The gene which encodes the first enzyme in the 2,4-D degradation pathway (FIG. 1) is tfdA. This gene encodes a monoxygenase which catalyzes the conversion of 2,4-D to 2,4-dichlorophenol (DCP). Transgenic tobacco and cotton plants containing the tfdA gene have been shown to have increased tolerance to 2,4-D (Streber et al., Bio/technology 7:811-816 (1989) and Bayley et al., Theor. Appl. Genet. 83:645-649 (1992), respectively). Until now, attempts to use the tfdA gene as a selectable marker to identify transformed plants have failed. Streber et at., Bio/technology 7:811-816 (1989) theorizes that "in a chimeric tissue transformed cells cannot develop into shoots because they are overgrown or otherwise inhibited by untransformed callus that is rapidly developing in the presence of 2,4-D."